To print a continuous tone image, customarily a halftoning process is applied to the image. Error diffusion, also indicated as “ED” in this document, is a well-established halftoning technique, especially suitable for printers that are able to produce dispersed dots, such as ink-jet printers.
Several error diffusion algorithms exist, that may be used for grey scale and for color. One way to extend an algorithm from grey scale to color is to apply the grey-scale algorithm separately on each color component.
In general, three methods exist to achieve a grey level in an image: area modulated printing, density modulated printing, and the combination of both. In area modulated printing, gray levels are achieved by applying a marking particle (such as ink or toner) to a smaller or larger area of a receiving substrate (such as paper). For ink-jet, this may be achieved by applying a larger quantity of ink, i.e. applying a larger drop size. In density modulated printing, grey levels are achieved by applying different types of marking particles to the receiving substrate, each type having a different density. For ink-jet, different ink densities (such as e.g. light cyan ink and dark cyan ink) may be used.
More background information on ED can be found in the following three patents, herein each incorporated by reference in their entirety for background information only.
U.S. Pat. No. 4,680,645 discloses multilevel error diffusion for a grayscale printing device using multiple drop sizes.
U.S. Pat. No. 5,975,671 discloses an error diffusion method for a printing device having multiple ink densities and dot sizes (or drop sizes); the ED levels are translated by a LUT (look-up table) to density/drop size combinations. More information on multilevel error diffusion in general can be found in this patent.
U.S. Pat. No. 5,963,714 discloses a multicolor and mixed-mode halftoning method, wherein a printer driver operates a printer capable of multiple dot-placement geometries or resolutions and multiple inks per color channel. The halftoning includes error diffusion.
In ordinary bilevel ED a gray value is quantized by means of thresholding. If the gray value is larger than the chosen threshold, the value is quantized to 1, otherwise it is quantized to 0. The quantisation error is distributed over neighboring unprocessed pixels. Therefore not the original pixel value is thresholded, but a modified pixel value which is equal to the original pixel value plus the quantization error the pixel has received from other, already processed, pixels. This feedback mechanism guarantees that the right mean tone value is produced.
In multilevel ED, the thresholding procedure is replaced by a more general quantization scheme.
The simplest quantization scheme is to quantize the modified pixel value to the nearest quantization level. The quantization error is once again fed to neighboring unprocessed pixels. The more quantization levels, the smaller the quantization error, and the better the quality of the ED image.
The multilevels are realized in practice in ink-jet printers by using inks of different densities, multiple drop sizes (in this document, the drop size is the amount of ink applied to a given pixel), or a combination of both. Error diffusion may also be applied to other printing techniques than ink-jet. Thus, in general, a level may be realized by a combination of a dot area (cf. the area modulated printing discussed above) and a dot density (cf. density modulated printing as discussed above). Some multilevel printing devices may provide such combinations wherein the dot area is kept constant; others may provide combinations wherein the dot density is kept constant; still others may provide combinations wherein both the dot area and the dot density may vary.
In case of a binary ink-jet printer using a light ink and a dark ink we can use a multilevel error diffusion with 3 levels: level 0=no drop of ink, level 1=a drop of light ink, level 2=a drop of dark ink.
Another example of an ink-jet printer with 6 levels: level 0=no drop of ink, level 1=a small drop of light ink, level 2=a medium drop of light ink, level 3=a large drop of light ink, level 4=a medium drop of dark ink, level 5=a large drop of dark ink.
While printing with multiple drop sizes is most often implemented in this way, printing with multiple ink densities is more often implemented by applying a set of inksplit curves, a set of lookup tables transforming each gray value into a set of gray values, one for each ink (e.g. light and dark ink). The thus obtained channels are processed independently by error diffusion algorithms to produce the output image channels. This method provides more control over how much light and dark ink is used to produce a certain tone value than the multilevel ED method. The inksplit method is chosen because the extra control on the ink quantities can be used to eliminate drying problems and image artifacts such as banding.